OG－5－25

wangyu73cn

这道题挺好，正说明了ETS的细致之处。不过现在没时间，我有空一定回复，对不起古镯了，现在帮着顶一下吧。

不过好像以前的讨论帖中看到过一些解释，有点道理，如有空就看看。

[此贴子已经被作者于2004-11-9 20:57:29编辑过]

wangyu73cn

25.

1、E is the best answer.

Best

2、The second and third paragraphs of the passage indicate that morphogenetic determinants are substances in the embryo that are activated after the egg has been fertilized and that “tell a cell what to become” (lines 21-23).

这句是说，先定位morphogenetic determinants，第二段，说他们一开始无活性，受精后，有活性了。从第一段知，有活性之后，形成的胚胎仍可以进行分割。也就是说受精和形成其最后定型的时间是不同的。所以选项C，inactive until the embryo cells become irreversibly committed to their final function这个有一定迷惑性的选项不对，因为时间点有误。

3、If, as the author asserts in the first paragraph, biologists have succeeded in dividing an embryo into two parts, each of which survives and develops into a normal embryo, it can be concluded that the quantity of morphogenetic determinants in the early embryo is greater than that required for the development of a single individual.

这是说，我们可以从生物学家分embryo的试验得知，morphogenetic determinants可用于多于一个的个体成长。这是一个简单的递进推理。

再看看其他选项，将杨继的话摘录下来：

25. It can be inferred from the passage that the morphogenetic determinants present in the early embryo are

(A)   located in the nucleus of the embryo cells

与原文不符，L38指出，位于细胞质中。

(B)   evenly distributed unless the embryo is not developing normally

L46－47明确说是不均匀的。

(C)   inactive until the embryo cells become irreversibly committed to their final function

胚胎细胞发挥其作用，就是这些morphogenetic determinants活跃并发挥作用的结果，见原文第三、四段。好像还是我在上边对第2句解释的分析更容易明白些。

(D)  identical to those that were already present in the unfertilized egg

文中无。我补充：至少一个是激活，一个是未激活的。可能有此一点就够了，不能说他们完全一样。

(E)   present in larger quantities than is necessary for the development of a single individual

如OG之解释。

以上请指正。

[此贴子已经被作者于2004-11-10 23:09:59编辑过]

wangyu73cn

以下是引用xealot在2005-3-11 9:43:00的发言：


偶觉得E也不十分好是因为，OG的说法只是通过第一段的那种特定embryo得出的结论；而显然的是原文的主要思想就是早期这种特定embryo的个别性。因此上面OG的说法偶觉得不特别准确...当然相对于其他选项好点儿...

^_^，我觉得“这种特定embryo的个别性”确实存在，但我有点不同意见，我认为：问题中 the morphogenetic determinants present in the early embryo 是不受这种个别性所影响的，E选项所述内容是完全准确地正确的。

wangyu73cn

我理解，你所说的个别性，是有的切割后的单体胚胎能够存活，有的单体胚胎不能存活。我所说不受影响，是说，只要切割得当，那胚胎完全是可以存活的。也就是说，只要切割得当，其形态基因决定物质是可以供多个胚胎正常存活的，因此，是能够满足一个以上的胚胎需要的。可能有误解之处，再探讨。
形象上，这有点像单胞胎和双胞胎，甚至多胞胎的形成机制。每个单胞胎蕴涵着发展成为双胞胎的可能。

[此贴子已经被作者于2005-3-11 13:23:33编辑过]

wangyu73cn

Nearly a century ago, biologists found that if they separated an invertebrate animal embryo into two parts at an early stage of its life, it would survive and develop as two normal embryos.
这句说，无脊椎动物胚胎可以分成两个正常胚胎。

This led them to believe that the cells in the early embryo are undetermined in the sense that each cell has the potential to develop in a variety of different ways.
这使他们相信胚胎中的每个细胞都有潜力以多种方式生长。

Later biologists found that the situation was not so simple. It matters in which plane the embryo is cut.
是否可以分成两个正常胚胎在于分割面。

If it is cut in a plane different from the one used by the early investigators, it will not form two whole embryos.
如果不以原先分割面进行切割，则不能形成两个完整的胚胎。

从上述来看，科学家们质疑第一句内容的焦点在于切割面，即是否能分成两个正常胚胎与分割面有关。而如果排除这种切割面的因素，象原来那样切割得当，就不能否定第一句中的命题了。当然，我同意楼上的观点，这是一个程度上的问题，而不是一个是与非的问题。E是最好的选项。

wangyu73cn

Studying sea urchins, biologist Paul Gross found

that an unfertilized egg contains substances that func-

tion as morphogenetic determinants. They are located

in the cytoplasm of the egg cell; i.e., in that part of the

(30) cell’s protoplasm that lies outside of the nucleus.

由上可知，morphogenetic determinants存在于cytoplasm之中，而不是cytoplasm(细胞质)

wangyu73cn

C is the best answer.

Lines 26-28 in from us that in his study of sea urchins, Gross “found that an unfertilized egg contains substances that function as morphogetic determinants.”
只说明了sea urchins的特殊情况。

Lines 39-40 assert that the “substances that Gross studied are maternal messenger RNA’s,” and in lines 41-42 we learn that these maternal messenger RNA’s can be found in “ a wide variety of organisms.”
这里将这种物质扩大到多种生命有机体中。也就是递进了一下。更加对应问题的题目。

注：我在上一个帖子中强调的是D选项的错误。

wangyu73cn

看来，cell determination是一个过程。文中也没有明确这个过程什么时候开始，至于何时结束，只知道在受精卵发展成胚胎，并且胚胎不可再分裂为两个单独活体时即结束了，这时基因决定物质(好像是MRNA，信使核糖核酸，文章中，它们是母性的)不均匀分布，所以胚胎不可任意分裂。

[此贴子已经被作者于2005-8-11 23:29:24编辑过]